Chromosome instability (and resulting aneuploidy) has been recognized as a hallmark of cancer cells, and contribute to multi-step carcinogenesis by facilitating the accumulation of genetic lesions required for acquisition of various malignant phenotypes. However, much remains to be learned in respect to the causes and mechanisms of chromosome instability in cancer. A number of studies have shown that abnormal amplification of centrosomes (generation of more than two centrosomes) occurs frequently in almost all types of human cancers, and there is a strong association between occurrence of centrosome amplification and aneuploidy. The detrimental consequence of centrosome amplification is prominently featured during mitosis by the formation of aberrant spindles organized by multiple centrosomes (spindle poles), leading to an increased frequency of chromosome segregation errors. Indeed, centrosome amplification is now widely accepted as one of the major factors that contribute to chromosome instability in cancer. Centrosome amplification occurs by several mechanisms, among which uncontrolled duplication of centrosomes is perhaps the leading cause. We and others have previously shown that loss or inactivating mutation of p53 tumor suppressor protein results in dysregulation of centrosome duplication, resulting in a high frequency of centrosome amplification, which undoubtedly contributes to the overall tumor suceptibility phenotype associated with loss or mutation of p53. During the current grant period, we have focused on how p53 is involved in the regulation of centrosome duplication, and we have made many critical findings for further understanding of the molecular mechanisms underlying this important cellular event. In this application, in addition to our continuing efforts to elucidate the molecular mechanism of how p53 itself regulates centrosome duplication and how loss of p53 leads to centrosome amplification, we will extend our studies to the upstream molecules/events of p53. This is critical to fully understand the p53-dependent regulation of duplication and numeral homeostasis of centrosomes as a cellular phenomenon rather than a "single-out" biological event. The studies proposed here will not only provide vital information for fully grasping the tumor suppressor activities of p53 and the role of p53 mutation in carcinogenesis, but also lead to effective cancer intervention protocols targeting centrosome duplication. Such an approach may prove effective in cancer intervention, since centrosome duplication, like DNA synthesis, is restricted to proliferating cells. Moreover, targeted inhibition of centrosome duplication will not only block cell division, but also suppress chromosome instability.